The present invention relates to a device for determining the phase errors of electromagnetic waves which are generated by a light source and transmitted by an optical instrument. Said device applies more particularly to the case in which the light source is an extended one.
Phase errors of this type are:
either continuous, for example when they are generated by constant deformations of an optical element, such as a mirror; PA1 or time-varying, for example when they are generated by atmospheric turbulence. PA1 to evaluate the optical qualities of the optical instrument in question, for example a telescope; or PA1 to determine the control inputs of a correction means, for example a deformable mirror, which is arranged on the path of said electromagnetic waves and is intended to correct said phase errors. PA1 a reception system which comprises a measurement plane provided with a plurality of measurement zones having photodiodes capable of measuring the intensity of the received light. To this end, said reception system includes a battery of lenses which focus the electromagnetic waves on to said photodiodes; PA1 a collimation lens for sending to said measurement plane the electromagnetic waves transmitted by said optical instrument; PA1 a defocusing system consisting of a lens or a mirror with variable focal length and capable of sending at least one pair of associated planes, which are conjugate with the pupil plane of the optical instrument, to said reception system for them to be measured; and PA1 a computation unit which determines said phase errors on the basis of measurements taken by said reception system on said associated planes. PA1 a reception system which comprises at least one measurement plane provided with a plurality of measurement zones which can measure the intensity of the received light; PA1 a collimation means for sending to said measurement plane the electromagnetic waves transmitted by said optical instrument; PA1 a defocusing system which can send at least one pair of associated planes, which are conjugate with the pupil plane of the optical instrument, to said reception system for them to be measured; and PA1 a computation unit which determines said phase errors on the basis of measurements taken by said reception system on said associated planes, is noteworthy in that it further includes a spatial filter which is arranged in the image plane of the optical instrument and is formed in such a way as to restrict the area of the light source seen by said reception system, while allowing the spatial spectrum of said phase errors to be transmitted. PA1 in a first embodiment, using a matrix calculation; and PA1 in a second embodiment, using a neurone-type calculation, which makes it possible to take account of nonlinearities.
Knowledge of such phase errors of electro-magnetic waves may, in particular, be used:
Many devices are known which can determine such phase errors of electromagnetic waves.
A first device, which is known by the name "Shack Hartmann", includes for this purpose a battery of microlenses which generate images of the source on a receiver of the charge coupled device type. With a device of this type, it is sufficient to calculate the centers of mass of said images and determine their displacements relative to a fixed origin.
However, although effective for point sources, this device is ill-suited to extended sources, and the advocated image processing is difficult to carry out in real-time.
A second known device determining phase errors is a differential interferometer. An interferometer of this type uses an image of the pupil of the optical instrument and includes a birefringent optical element which splits said image of the pupil. The two partial images which are obtained are phase-shifted by about a quarter wavelength. Said differential interferometer further includes means for measuring the illuminations resulting from the interferences produced between these two partial images, the illuminations being proportional to the local slope of the deformations of the wavefront, which makes it possible to determine said phase errors.
However, this known device does not make it possible to measure all types of phase errors, since ambiguities arise in the measurements, in particular when said local slopes are too great.
A third known device is a curvature analyzer, for example such as the one described in an article by Roddier, Northcott and Graves, entitled "A simple low-order adaptive optics system for near-infrared applications" published in the journal "Publications of the Astronomical Society of the Pacific" in January 1991. A curvature analyzer of this type includes:
The device presented above is satisfactory, both in terms of its implementation and in terms of the accuracy of its measurements, when the electromagnetic waves to be analyzed are generated by a point source or a spatially limited source. However, this device cannot be used for extended sources.